Lower-deck promotion and the Victorian exception

When did Royal Navy promotion and training of petty officers begin to overcome class barriers?

The tradition was centuries old; the Victorian era was the unusually restrictive period rather than the starting point for mobility.

Sailors and merchant seamen could rise through petty-officer and commissioned ranks, sometimes reaching admiral. The Victorian Navy made such movement markedly harder because of its social criteria. Fisher criticized those restrictions by observing that Nelson could not have entered under contemporary rules. Mobility became more normal again outside that period.